Designers, manufacturers, and users of electronic computers and computing systems require reliable and efficient equipment for storage and retrieval of information in digital form. Conventional storage systems, such as magnetic disk drives, are typically utilized for this purpose and are well known in the art. However, the amount of information that is digitally stored continually increases, and designers and manufacturers of magnetic recording media work to increase the storage capacity of magnetic disks.
Referring to prior art FIG. 1, shown therein is a plan view of a portion of a high areal recording density bit patterned data/information storage and retrieval system 10 designed for random access operation, comprising a bit patterned medium 12, a write pole 14 of a write head (illustratively trapezoidal wedge-shaped), and a reader sensor 16. The respective opposing directions of relative movement of the medium and write pole are indicated by the arrows in the figure. The bit patterned medium 12 comprises a plurality of spaced apart magnetic elements 18 or bits of given type, e.g., longitudinal, perpendicular, multi-layer, etc., illustratively circularly-shaped “dots” arranged in a regular array on a non-magnetic substrate, i.e., in concentric rings 36 n, n+1. n+2. n+3. . . . comprised of substantially equally spaced apart dots extending in a down-ring direction, and radially extending rows 44a, b, c. . . . comprised of substantially equally spaced apart dots extending in a cross-ring direction.
Still referring to FIG. 1, it is seen that the narrower end 20 of the trapezoidal, wedge-shaped write pole 14 forms a leading edge of the write pole 14 when the latter is moved relative to medium 12, and the wider end 22 of the trapezoidal, wedge-shaped write pole 14 forms a trailing edge of the write pole 14. If the write pole of the write head is configured in conformity with the above-described requirement for the width of the write pole to be scaled down in relation to the track and bit density of the media, and therefore the maximum width of the write pole is the same as, or smaller than, the dot pitch, i.e., the spacing between dots in adjacent rings or tracks. As may be apparent, according to this scheme the concentric rings 36 n, n+1. n+2. n+3 . . . may be accessed and written to in any desired order or pattern, i.e., randomly. For example, if 4 bits were written to 6 rings in random fashion, a writing sequence could be represented as: 1111 3333 2222 4444 6666 5555. FIG. 1 illustrates writing in a random fashion, as shown by pass 1 24, pass 2 26, and pass 3 28.
In conventional magnetic disk data/information storage, the data/information is stored in a continuous magnetic thin film overlying a substantially rigid, non-magnetic disk. Each bit of data/information is stored by magnetizing a small area of the thin magnetic film using a magnetic transducer (write head) that provides a sufficiently strong magnetic field to effect a selected alignment of the small area (magnetic grain) of the film. The magnetic moment, area, and location of the small area comprise a bit of binary information which must be precisely defined in order to allow a magnetic read head to retrieve the stored data/information.
Such conventional magnetic disk storage media incur drawbacks and disadvantages which adversely affect realization of high areal density data/information storage, as follows:
(1) the boundaries between adjacent pairs of bits tend to be ragged in continuous magnetic films, resulting in noise generation during reading; and
(2) the requirement for increased areal recording density has necessitated a corresponding decrease in recording bit size or area. Consequently, grain sizes of continuous film media have become extremely minute, e.g., on the order of nanometers (nm). In order to obtain a sufficient output signal to noise ratio from such minute bits, the saturation magnetization (Ms) should be sufficiently large and media grain size as small as possible. However, if the grain size is extremely small the loss of the magnetization of such minute bits results in a loss of stored information due to magnetization thermal fluctuation reversal, also known as the “superparamagnetic effect”.
The superparamagnetic effect is a major limiting factor in increasing the areal density of continuous film magnetic recording media. Superparamagnetism results from thermal excitations which perturb the magnetization of grains in a ferromagnetic material, resulting in unstable magnetization. So-called “patterned” or “bit patterned” magnetic media (“BPM”) have been proposed as a means for overcoming the magnetization reversal via the superparamagnetic effect, e.g., as disclosed in U.S. Pat. No. 5,956,216. The term “patterned” media generally refers to magnetic data/information storage and retrieval media wherein a plurality of discrete, independent regions of magnetic material form discrete, independent magnetic elements which function as recording bits are formed on a non-magnetic substrate. Since the regions of ferromagnetic material comprising the magnetic bits or elements are independent of each other, mutual interference between neighboring bits can be minimized. As a consequence, patterned magnetic media are advantageous vis-à-vis continuous magnetic media in terms of thermal stability and jitter noise from neighboring magnetic bits.
Generally, each magnetic bit or element has the same size and shape, e.g., circularly shaped “dots”, and is composed of the same magnetic material as the other bits or elements. The bits or elements are arranged in a regular pattern over the substrate surface, with each bit or element having a small size and desired magnetic anisotropy, so that, in the absence of an externally applied magnetic field, the magnetic moments of each discrete magnetic bit or element will be aligned along the same magnetic easy axis. Stated differently, the magnetic moment of each discrete magnetic bit or element has only two states; the same in magnitude but aligned in opposite directions. Each discrete magnetic bit or element forms a single magnetic domain and the size, area, and location of each domain is determined during the fabrication process.
During writing operation of bit patterned media, the direction of the magnetic moment of the single magnetic domain element or bit is flipped along the easy axis, and during reading operation, the direction of the magnetic moment of the single magnetic domain element or bit is sensed. While the direction of the magnetic easy axis of each of the magnetic domains, elements, or bits can be parallel or perpendicular to the surface of the domain, element, or bit, corresponding to conventional continuous longitudinal and perpendicular media, respectively, patterned media comprised of domains, elements, or bits with perpendicularly oriented magnetic easy axis are advantageous in achieving higher areal recording densities for the reasons given above.
Notwithstanding the substantial increase in recording/data storage performance capability afforded by bit patterned media (BPM) vis-a-vis conventional continuous film-based media, the escalating requirement for even higher areal recording densities engenders a significant problem in writing data/information to ultra-high areal recording density media arising from limitation of the available write head field at very high track densities, e.g., >˜300 ktpi. From a solid angle viewpoint, it is evident that the available recording (write) field decreases as track density increases. The primary reason for this effect is the requirement for reduction in the pole width of the write head, necessitated by the reduction in track spacing (pitch) and bit size in ultra-high areal recording density media, leading to a corresponding reduction in the total write field applied to the media. In addition to the requirement for reduction in the write pole width as track density increases, the length of the write pole must be reduced in order to mitigate problems arising when the write head is at skew. Disadvantageously, however, the head-to-media spacing (“HMS”) and media thickness cannot be scaled down to the same extent as the write pole width in order to remedy or at least mitigate the aforementioned problem.
In view of the foregoing, there exists a clear need for improved systems and methodology for facilitating accurate writing to media with very high to ultra-high areal recording densities.